Fiber's Optics. Technician's Manual

Here's an indispensable tool for all technicians and electricians who need to learn about optimal fiber optic design and installation as well as pick up the latest troubleshooting tips and techniques. The information presented in Fiber Optics Technician's Manual has been refined and updated based on feedback and participation from fiber optics technicians like you. Now you can have this valuable information at your fingertips, available to you when and where you need it. Following a concise overview of the basics and history of fiber optics, every crucial area is covered in its practical aspects including networks, cables, cable specifications, connectors and splices, and fiber optic hardware. You will learn about proven guidelines for better fiber optic design and installation, the key to winning contracts through more accurate estimating and bidding, ways of ensuring safety on the job, as well as proper documentation, restoration, and testing procedures. A handy glossary clarifies even the most difficult technical terms, while a standards section points out the regulations governing the field.

You may be interested in

You can write a book review and share your experiences. Other readers will always be interested in your opinion of the books you've read. Whether you've loved the book or not, if you give your honest and detailed thoughts then people will find new books that are right for them.

C H A P T E R
1
THE ORIGINS OF
FIBER OPTIC
COMMUNICATIONS
JEFF HECHT
Optical communication systems date back two centuries, to the “optical telegraph” invented by French engineer Claude Chappe in the 1790s. His system was
a series of semaphores mounted on towers, where human operators relayed messages from one tower to the next. It beat hand-carried messages hands down, but
by the mid-19th century it was replaced by the electric telegraph, leaving a scattering of “telegraph hills” as its most visible legacy.
Alexander Graham Bell patented an optical telephone system, which he
called the Photophone, in 1880, but his earlier invention, the telephone, proved
far more practical. He dreamed of sending signals through the air, but the
atmosphere did not transmit light as reliably as wires carried electricity. In the
decades that followed, light was used for a few special applications, such as signaling between ships, but otherwise optical communications, such as the experimental Photophone Bell donated to the Smithsonian Institution, languished on
the shelf.
Thanks to the Alfred P. Sloan Foundation for research support. This is a much expanded
version of an article originally published in the November 1994 Laser Focus World.
1
2
CHAPTER 1 — THE ORIGINS OF FIBER OPTIC COMMUNICATIONS
In the intervening years, a new technology that would ultimately solve the
problem of optical transmission slowly took root, although it was a long time
before it was adapted for communications. This technology depended on the phenomenon of total internal reflection, which can confine light in a material surrounded by other materials with lower refractive index, such as glass in air.
In the 1840s, Swiss physicist Daniel Collodon and French physicist Jacques
Babinet showed that light could be guided along jets of water for fountain displays. British physicist John Tyndall popularized light guiding in a demonstration
he first used in 1854, guiding light in a jet of water flowing from a tank. By the
turn of the century, inventors realized that bent quartz rods could carry light and
patented them as dental illuminators. By the 1940s, many doctors used illuminated Plexiglas tongue depressors.
Optical fibers went a step further. They are essentially transparent rods of
glass or plastic stretched to be long and flexible. During the 1920s, John Logie
Baird in England and Clarence W. Hansell in the United States patented the idea
of using arrays of hollow pipes or transparent rods to transmit images for television or facsimile systems. However, the first person known to have demonstrated
image transmission through a bundle of optical fibers was Heinrich Lamm (Figure 1-1), then a medical student in Munich. His goal was to look inside inaccessi-
Figure 1-1 Heinrich Lamm as a German
medical student in 1929, about the time
he made the first bundle of fibers to
transmit an image. Courtesy Michael
Lamm
CHAPTER 1 — THE ORIGINS OF FIBER OPTIC COMMUNICATIONS
3
Figure 1-2 Holger Møller Hansen in his workshop.
Courtesy Holger Møller Hansen
ble parts of the body, and in a 1930 paper he reported transmitting the image of
a light bulb filament through a short bundle. However, the unclad fibers transmitted images poorly, and the rise of the Nazis forced Lamm, a Jew, to move to
America and abandon his dreams of becoming a professor of medicine.
In 1951, Holger Møller Hansen (Figure 1-2) applied for a Danish patent on
fiber optic imaging. However, the Danish patent office denied his application, citing the Baird and Hansell patents, and Møller Hansen was unable to interest
companies in his invention. Nothing more was reported on fiber bundles until
1954, when Abraham van Heel (Figure 1-3), of the Technical University of Delft
4
CHAPTER 1 — THE ORIGINS OF FIBER OPTIC COMMUNICATIONS
Figure 1-3 Abraham C. S. van Heel,
who made clad fibers at the Technical
University of Delft. Courtesy H. J.
Frankena, Faculty of Applied Physics,
Technical University of Delft
Figure 1-4 Harold H. Hopkins looks into
an optical instrument that he designed.
Courtesy Kelvin P. Hopkins
in Holland, and Harold H. Hopkins (Figure 1-4) and Narinder Kapany, of Imperial College in London, separately announced imaging bundles in the prestigious
British journal Nature.
Neither van Heel nor Hopkins and Kapany made bundles that could carry
light far, but their reports began the fiber optics revolution. The crucial innovation was made by van Heel, stimulated by a conversation with the American optical physicist Brian O’Brien (Figure 1-5). All earlier fibers were bare, with total
internal reflection at a glass-air interface. Van Heel covered a bare fiber of glass
or plastic with a transparent cladding of lower refractive index. This protected
the total-reflection surface from contamination and greatly reduced crosstalk
between fibers. The next key step was development of glass-clad fibers by
Lawrence Curtiss (Figure 1-6), then an undergraduate at the University of Michigan working part-time on a project with physician Basil Hirschowitz (Figure 1-7)
and physicist C. Wilbur Peters to develop an endoscope to examine the inside of
the stomach (Figure 1-8). Will Hicks, then working at the American Optical Co.,
CHAPTER 1 — THE ORIGINS OF FIBER OPTIC COMMUNICATIONS
5
Figure 1-5 Brian O’Brien, who suggested
that cladding would guide light along fiber.
Courtesy Brian O’Brien, Jr.
made glass-clad fibers at about the same time, but his group lost a bitterly contested patent battle. By 1960, glass-clad fibers had attenuation of about one decibel per meter, fine for medical imaging, but much too high for communications.
Meanwhile, telecommunications engineers were seeking more transmission
bandwidth. Radio and microwave frequencies were in heavy use, so engineers
looked to higher frequencies to carry the increased loads they expected with the
growth of television and telephone traffic. Telephone companies thought video
telephones lurked just around the corner and would escalate bandwidth demands
even further. On the cutting edge of communications research were millimeterwave systems, in which hollow pipes served as waveguides to circumvent poor
atmospheric transmission at tens of gigahertz, where wavelengths were in the
millimeter range.
Even higher optical frequencies seemed a logical next step in 1958 to Alec
Reeves, the forward-looking engineer at Britain’s Standard Telecommunications
Laboratories, who invented digital pulse-code modulation before World War II.
Other people climbed on the optical communications bandwagon when the laser
6
CHAPTER 1 — THE ORIGINS OF FIBER OPTIC COMMUNICATIONS
Figure 1-6 Lawrence Curtiss, with the equipment he used to make glass-clad
fibers at the University of Michigan. Courtesy University of Michigan News and
Information Services Records, Bentley Historical Library, University of Michigan
CHAPTER 1 — THE ORIGINS OF FIBER OPTIC COMMUNICATIONS
Figure 1-7 Basil Hirschowitz about
the time he helped to develop the first
fiber optic endoscope. Courtesy
Basil Hirschowitz
Figure 1-8 Prototype fiber optic endoscope made by Lawrence
Curtiss, Wilbur Peters, and Basil Hirschowitz at the University of
Michigan. Courtesy Basil Hirschowitz
7
8
CHAPTER 1 — THE ORIGINS OF FIBER OPTIC COMMUNICATIONS
was invented in 1960. The July 22, 1960, issue of Electronics introduced its
report on Theodore Maiman’s demonstration of the first laser by saying, “Usable
communications channels in the electromagnetic spectrum may be extended by
development of an experimental optical-frequency amplifier.”
Serious work on optical communications had to wait for the CW heliumneon laser. While air is far more transparent to light at optical wavelengths than
to millimeter waves, researchers soon found that rain, haze, clouds, and atmospheric turbulence limited the reliability of long-distance atmospheric laser links.
By 1965, it was clear that major technical barriers remained for both millimeterwave and laser telecommunications. Millimeter waveguides had low loss,
although only if they were kept precisely straight; developers thought the biggest
problem was the lack of adequate repeaters. Optical waveguides were proving to
be a problem. Stewart Miller’s group at Bell Telephone Laboratories was working on a system of gas lenses to focus laser beams along hollow waveguides for
long-distance telecommunications. However, most of the telecommunications
industry thought the future belonged to millimeter waveguides.
Optical fibers had attracted some attention because they were analogous in
theory to plastic dielectric waveguides used in certain microwave applications. In
1961, Elias Snitzer at American Optical, working with Hicks at Mosaic Fabrications (now Galileo Electro-Optics), demonstrated the similarity by drawing fibers
with cores so small they carried light in only one waveguide mode. However, virtually everyone considered fibers too lossy for communications; attenuation of a
decibel per meter was fine for looking inside the body, but communications operated over much longer distances and required loss of no more than 10 or 20 decibels per kilometer.
One small group did not dismiss fibers so easily—a team at Standard
Telecommunications Laboratories (STL), initially headed by Antoni E. Karbowiak, that worked under Reeves to study optical waveguides for communications. Karbowiak soon was joined by a young engineer born in Shanghai, Charles
K. Kao (Figure 1-9).
Kao took a long, hard look at fiber attenuation. He collected samples from
fiber makers, and carefully investigated the properties of bulk glasses. His
research convinced him that the high losses of early fibers were due to impurities,
not to silica glass itself. In the midst of this research, in December 1964, Karbowiak left STL to become chair of electrical engineering at the University of
New South Wales in Australia, and Kao succeeded him as manager of optical
communications research. With George Hockham (Figure 1-10), another young
STL engineer who specialized in antenna theory, Kao worked out a proposal for
long-distance communications over singlemode fibers. Convinced that fiber loss
should be reducible below 20 decibels per kilometer, they presented a paper at a
London meeting of the Institution of Electrical Engineers (IEE). The April 1,
1966, issue of Laser Focus noted Kao’s proposal:
CHAPTER 1 — THE ORIGINS OF FIBER OPTIC COMMUNICATIONS
Figure 1-9 Charles K. Kao making optical measurements at Standard
Telecommunications Laboratories. Courtesy BNR Europe
At the IEE meeting in London last month, Dr. C. K. Kao observed that
short-distance runs have shown that the experimental optical waveguide
developed by Standard Telecommunications Laboratories has an information-carrying capacity . . . of one gigacycle, or equivalent to about
200 tv channels or more than 200,000 telephone channels. He described
STL’s device as consisting of a glass core about three or four microns in
diameter, clad with a coaxial layer of another glass having a refractive
index about one percent smaller than that of the core. Total diameter of
the waveguide is between 300 and 400 microns. Surface optical waves
are propagated along the interface between the two types of glass.
According to Dr. Kao, the fiber is relatively strong and can be easily
supported. Also, the guidance surface is protected from external influences. . . . the waveguide has a mechanical bending radius low enough to
9
10
CHAPTER 1 — THE ORIGINS OF FIBER OPTIC COMMUNICATIONS
Figure 1-10 George Hockham with the metal waveguides he made to model
waveguide transmission in fibers. Courtesy BNR Europe
make the fiber almost completely flexible. Despite the fact that the best
readily available low-loss material has a loss of about 1000 dB/km, STL
believes that materials having losses of only tens of decibels per kilometer will eventually be developed.
Kao and Hockham’s detailed analysis was published in the July 1966, Proceedings of the Institution of Electrical Engineers. Their daring forecast that fiber
loss could be reduced below 20 dB/km attracted the interest of the British Post
Office, which then operated the British telephone network. F.F. Roberts, an engineering manager at the Post Office Research Laboratory (then at Dollis Hill in
London), saw the possibilities and persuaded others at the Post Office. His boss,
Jack Tillman, tapped a new research fund of 12 million pounds to study ways to
decrease fiber loss.
With Kao almost evangelically promoting the prospects of fiber communications, and the Post Office interested in applications, laboratories around the
world began trying to reduce fiber loss. It took four years to reach Kao’s goal of
20 dB/km, and the route to success proved different than many had expected.
Most groups tried to purify the compound glasses used for standard optics,
which are easy to melt and draw into fibers. At the Corning Glass Works (now
CHAPTER 1 — THE ORIGINS OF FIBER OPTIC COMMUNICATIONS
11
Corning, Inc.), Robert Maurer, Donald Keck, and Peter Schultz (Figure 1-11)
started with fused silica, a material that can be made extremely pure, but has a
high melting point and a low refractive index. They made cylindrical preforms by
depositing purified materials from the vapor phase, adding carefully controlled
levels of dopants to make the refractive index of the core slightly higher than that
of the cladding, without raising attenuation dramatically. In September 1970,
they announced they had made singlemode fibers with attenuation at the 633nanometer (nm) helium neon line below 20 dB/km. The fibers were fragile, but
tests at the new British Post Office Research Laboratories facility in Martlesham
Heath confirmed the low loss.
The Corning breakthrough was among the most dramatic of many developments that opened the door to fiber optic communications. In the same year, Bell
Labs and a team at the Loffe Physical Institute in Leningrad (now St. Petersburg)
made the first semiconductor diode lasers able to emit carrier waves (CW) at
room temperature. Over the next several years, fiber losses dropped dramatically,
aided both by improved fabrication methods and by the shift to longer wavelengths where fibers have inherently lower attenuation.
Figure 1-11 Donald Keck, Robert Maurer, and Peter Schultz (left to right), who
made the first low-loss fibers in 1970 at Corning. Courtesy Corning, Incorporated
12
CHAPTER 1 — THE ORIGINS OF FIBER OPTIC COMMUNICATIONS
Early singlemode fibers had cores several micrometers in diameter and in the
early 1970s that bothered developers. They doubted it would be possible to
achieve the micrometer-scale tolerances needed to couple light efficiently into the
tiny cores from light sources or in splices or connectors. Not satisfied with the
low bandwidth of step-index multimode fiber, they concentrated on multimode
fibers with a refractive-index gradient between core and cladding, and core diameters of 50 or 62.5 micrometers. The first generation of telephone field trials in
1977 used such fibers to transmit light at 850 nm from gallium-aluminumarsenide laser diodes.
Those first-generation systems could transmit light several kilometers without repeaters, but were limited by loss of about 2 dB/km in the fiber. A second
generation soon appeared, using new indium gallium arsenide phosphide
(InGaAsP) lasers that emitted at 1.3 micrometers, where fiber attenuation was as
low as 0.5 dB/km, and pulse dispersion was somewhat lower than at 850 nm.
Development of hardware for the first transatlantic fiber cable showed that singlemode systems were feasible, so when deregulation opened the long-distance
phone market in the early 1980s, the carriers built national backbone systems of
singlemode fiber with 1300-nm sources. That technology has spread into other
telecom applications and remains the standard for most fiber systems.
However, a new generation of singlemode systems is now beginning to find
applications in submarine cables and systems serving large numbers of subscribers. They operate at 1.55 micrometers, where fiber loss is 0.2 to 0.3 dB/km,
allowing even longer repeater spacings. More important, erbium-doped optical
fibers can serve as optical amplifiers at that wavelength, avoiding the need for
electro-optic regenerators. Submarine cables with optical amplifiers can operate
at speeds to 5 gigabits per second and can be upgraded from lower speeds simply
by changing terminal electronics. Optical amplifiers also are attractive for fiber
systems delivering the same signals to many terminals, because the fiber amplifiers can compensate for losses in dividing the signals among many terminals.
The biggest challenge remaining for fiber optics is economic. Today telephone and cable television companies can cost justify installing fiber links to
remote sites serving tens to a few hundreds of customers. However, terminal
equipment remains too expensive to justify installing fibers all the way to homes,
at least for present services. Instead, cable and phone companies run twisted wire
pairs or coaxial cables from optical network units to individual homes. Time will
see how long that lasts.
CHAPTER 1 — THE ORIGINS OF FIBER-OPTIC COMMUNICATIONS
13
REVIEW QUESTIONS
1. Confining light in a material by surrounding it by another material with
lower refractive index is the phenomenon of _____________
a. cladding.
b. total internal reflection.
c. total internal refraction.
d. transmission.
2. Abraham van Heel, in order to increase the total internal reflection, covered bare fiber with transparent cladding of _____________
a. higher refractive index.
b. lower refractive index.
c. higher numerical aperture.
d. lower numerical aperture.
3. The high loss of early optical fiber was mainly due to _____________
a. impurities.
b. silica.
c. wave guides.
d. small cores.
4. _____________, using fused silica, made the first low loss (<20 dB/Km)
singlemode optical fiber.
a. Standard Telecommunications Laboratory
b. The Post Office Research Laboratory
c. Corning Glass Works
d. Dr. Charles K. Kao
5. Erbium-doped optical fiber can serve as _____________
a. cladding.
b. a pulse suppresor.
c. a regenerator.
d. an amplifier.
C H A P T E R
2
BASICS OF
FIBER OPTICS
E L I A S A. AW A D
INTRODUCTION
Optical fiber is the medium in which communication signals are transmitted from
one location to another in the form of light guided through thin fibers of glass or
plastic. These signals are digital pulses or continuously modulated analog streams
of light representing information. These can be voice information, data information, computer information, video information, or any other type of information.
These same types of information can be sent on metallic wires such as twisted
pair and coax and through the air on microwave frequencies. The reason to use
optical fiber is because it offers advantages not available in any metallic conductor or microwaves.
The main advantage of optical fiber is that it can transport more information
longer distances in less time than any other communications medium. In addition,
it is unaffected by the interference of electromagnetic radiation, making it possible
to transmit information and data with less noise and less error. There are also
many other applications for optical fiber that are simply not possible with metallic conductors. These include sensors/scientific applications, medical/surgical
applications, industrial applications, subject illumination, and image transport.
Most optical fibers are made of glass, although some are made of plastic. For
mechanical protection, optical fiber is housed inside cables. There are many types
15
16
CHAPTER 2 — BASICS OF FIBER OPTICS
and configurations of cables, each for a specific application: indoor, outdoor, in
the ground, underwater, deep ocean, overhead, and others.
An optical fiber data link is made up of three elements (Figure 2-1):
1. A light source at one end (laser or light-emitting diode [LED]), including
a connector or other alignment mechanism to connect to the fiber. The
light source will receive its signal from the support electronics to convert
the electrical information to optical information.
2. The fiber (and its cable, connectors, or splices) from point to point. The
fiber transports this light to its destination.
3. The light detector on the other end with a connector interface to the
fiber. The detector converts the incoming light back to an electrical signal, producing a copy of the original electrical input. The support electronics will process that signal to perform its intended communications
function.
The source and detector with their necessary support electronics are called the
transmitter and receiver, respectively.
Transmitter
Source
Driver
LED or Laser
Input
Connectors
Receiver
Preamp/Trigger
Photodiode
Output
Figure 2-1
A typical fiber optic data link.
Cables
CHAPTER 2 — BASICS OF FIBER OPTICS
Repeater
Repeater
Fiber
Figure 2-2
Repeater
Fiber
17
Repeater
Fiber
Long distance data links require repeaters to regenerate signals.
In long-distance systems (Figure 2-2) the use of intermediate amplifiers may
be necessary to compensate for the signal loss over the long run of the fiber.
Therefore, long-distance networks will be comprised of a number of identical
links connected together. Each repeater consists of a receiver, transmitter, and
support electronics.
OPTICAL FIBER
Optical fiber (Figure 2-3) is comprised of a light-carrying core surrounded by a
cladding that traps the light in the core by the principle of total internal reflection. By making the core of the fiber of a material with a higher refractive index,
we can cause the light in the core to be totally reflected at the boundary of the
cladding for all light that strikes at greater than a critical angle. The critical angle
is determined by the difference in the composition of the materials used in the
core and cladding. Most optical fibers are made of glass, although some are made
of plastic. The core and cladding are usually fused silica glass covered by a plastic coating, called the buffer, that protects the glass fiber from physical damage
and moisture. Some all-plastic fibers are used for specific applications.
Glass optical fibers are the most common type used in communication applications. Glass optical fibers can be singlemode or multimode. Most of today’s
telecom and community antenna television (CATV) systems use singlemode
fibers, whereas local area networks (LANs) use multimode graded-index fibers.
Core
Buffer Coating
Cladding
Figure 2-3
Optical fiber construction.
18
CHAPTER 2 — BASICS OF FIBER OPTICS
Singlemode fibers are smaller in core diameter than multimode fibers and offer
much greater bandwidth, but the larger core size of multimode fiber makes coupling to low cost sources such as LEDs much easier. Multimode fibers may be of
the step-index or graded-index design.
Plastic optical fibers are large core step-index multimode fibers, although
graded-index plastic fiber is under development. Because plastic fibers have a
large diameter and can be cut with simple tools, they are easy to work with and
can use low-cost connectors. Plastic fiber is not used for long distance because it
has high attenuation and lower bandwidth than glass fibers. However, plastic
optical fiber may be useful in the short runs from the street to the home or office
and within the home or office.
There are two basic types of optical fiber—multimode and singlemode (Figure 2-4). Multimode fiber means that light can travel many different paths (called
modes) through the core of the fiber, entering and leaving the fiber at various
angles. The highest angle that light is accepted into the core of the fiber defines
Multimode Step Index
Cladding
Core
Multimode Graded Index
Cladding
Core
Singlemode
Cladding
Core
Figure 2-4
The three types of optical fiber.
CHAPTER 2 — BASICS OF FIBER OPTICS
Table 2-1
Fiber Type
19
Fiber Types and Typical Specifications
Core/Cladding Attenuation Coefficient (dBkm)
Diameter(m)
850 nm
1300 nm
1550 nm
Multimode/Plastic
1 mm
Multimode/Step Index
200/240
Multimode/Graded Index 50/125
62.5/125
85/125
100/140
Singlemode
8-9/125
(1 dB/m
6
3
3
3
3
@665 nm)
1
1
1
1
0.5
0.3
Bandwidth
(MHz-km)
Low
50 @ 850 nm
600 @1300 nm
500 @1300 nm
500 @1300 nm
300 @1300 nm
high
the numerical aperture (NA). Two types of multimode fiber exist, distinguished
by the index profile of their cores and how light travels in them (Table 2-1).
Step-index multimode fiber has a core composed completely of one type of
glass. Light travels in straight lines in the fiber, reflecting off the core/cladding
interface. The NA is determined by the difference in the indices of refraction of
the core and cladding and can be calculated by Snell’s law. Since each mode or
angle of light travels a different path, a pulse of light is dispersed while traveling
through the fiber, limiting the bandwidth of step-index fiber.
In graded-index multimode fiber, the core is composed of many different layers of glass, chosen with indices of refraction to produce an index profile approximating a parabola, where from the center of the core the index of refraction gets
lower toward the cladding. Since light travels faster in the lower index of refraction glass, the light will travel faster as it approaches the outside of the core. Likewise, the light traveling closest to the core center will travel the slowest. A
properly constructed index profile will compensate for the different path lengths
of each mode, increasing the bandwidth capacity of the fiber by as much as 100
times over that of step-index fiber.
Singlemode fiber just shrinks the core size to a dimension about six times the
wavelength of light traveling in the fiber and it has a smaller difference in the
refractive index of the core and cladding, causing all the light to travel in only one
mode. Thus modal dispersion disappears and the bandwidth of the fiber increases
tremendously over graded-index fiber.
FIBER MANUFACTURE
Three methods are used today to fabricate moderate-to-low loss waveguide
fibers: modified chemical vapor deposition (MCVD), outside vapor deposition
(OVD), and vapor axial deposition (VAD).
20
CHAPTER 2 — BASICS OF FIBER OPTICS
Hollow Glass Preform
Gases
Rotating
Flame
Soot Deposited
Inside Tube
Heat Source Moving
Back and Forth
Figure 2-5
Modified chemical vapor deposition (MCVD).
Modified Chemical Vapor Deposition (MCVD)
In MCVD a hollow glass tube, approximately 3 feet long and 1 inch in diameter
(1 m long by 2.5 cm diameter), is placed in a horizontal or vertical lathe and spun
rapidly. A computer-controlled mixture of gases is passed through the inside of
the tube. On the outside of the tube, a heat source (oxygen/hydrogen torch) passes
up and down as illustrated in Figure 2-5.
Each pass of the heat source fuses a small amount of the precipitated gas
mixture to the surface of the tube. Most of the gas is vaporized silicon dioxide
(glass), but there are carefully controlled remounts of impurities (dopants) that
cause changes in the index of refraction of the glass. As the torch moves and the
preform spins, a layer of glass is formed inside the hollow preform. The dopant
(mixture of gases) can be changed for each layer so that the index may be varied
across the diameter.
After sufficient layers are built up, the tube is collapsed into a solid glass rod
referred to as a preform. It is now a scale model of the desired fiber, but much
shorter and thicker. The preform is then taken to the drawing tower, where it is
pulled into a length of fiber up to 10 kilometers long.
Outside Vapor Deposition (OVD)
The OVD method utilizes a glass target rod that is placed in a chamber and spun
rapidly on a lathe. A computer-controlled mixture of gases is then passed between
the target rod and the heat source as illustrated in Figure 2-6. On each pass of the
heat source, a small amount of the gas reacts and fuses to the outer surface of the
rod. After enough layers are built up, the target rod is removed and the remaining
soot preform is collapsed into a solid rod. The preform is then taken to the tower
and pulled into fiber.
CHAPTER 2 — BASICS OF FIBER OPTICS
21
Soot Preform
Rotating
Flame
Target Rod
Gases
Heat Source Moving
Back and Forth
Figure 2-6
Outside vapor deposition (OVD).
Vapor Axial Deposition (VAD)
The VAD process utilizes a very short glass target rod suspended by one end. A
computer-controlled mixture of gases is applied between the end of the rod and
the heat source as shown in Figure 2-7. The heat source is slowly backed off as
the preform lengthens due to tile soot buildup caused by gases reacting to the heat
and fusing to the end of the rod. After sufficient length is formed, the target rod
is removed from the end, leaving the soot preform. The preform is then taken to
the drawing tower to be heated and pulled into the required fiber length.
Target Rod
Soot Preform
Heat Sources
Moving Down
Gases
Figure 2-7
Vapor axial deposition (VAD).
22
CHAPTER 2 — BASICS OF FIBER OPTICS
Coating the Fiber for Protection
After the fiber is pulled from the preform, a protective coating is applied very
quickly after the formation of the hair-thin fiber (Figure 2-8). The coating is necessary to provide mechanical protection and prevent the ingress of water into any
fiber surface cracks. The coating typically is made up of two parts, a soft inner
coating and a harder outer coating. The overall thickness of the coating varies
between 62.5 and 187.5 µm, depending on fiber applications.
Moveable Blank Holder
Furnace
Preform
Fiber Drawing
Diameter Monitor
Coating Applicator
Ultraviolet Lamps
Screen Tester
Figure 2-8
Drawing the fiber from the preform and coating the fiber.
CHAPTER 2 — BASICS OF FIBER OPTICS
23
These coatings are typically strippable by mechanical means and must be
removed before fibers can be spliced or connectorized.
ADVANCED STUDY
What Is the Index of Refraction?
The index of refraction of a material is the ratio of the speed of light in vacuum to that in the material. In other words, the index of refraction is a
measure of how much the speed of light slows down after it enters the
material. Since light has its highest speed in vacuum, and since light
slows down whenever it enters any medium (water, plastic, glass, crystal,
oil, etc.), the index of refraction of all media is greater than one. For example, the index of refraction in a vacuum is 1, that of glass and plastic optical fibers is approximately 1.5, and water has an index of refraction of
approximately 1.3
When light goes from one material to another of a different index of
refraction, its path will bend, causing an illusion similar to the “bent” stick
stuck into water. At its limits, this phenomenon is used to reflect the light
at the core/cladding boundary of the fiber and trap it in the core (Figure
2-9). By choosing the material differences between the core and cladding,
one can select the angle of light at which this light trapping, called total
internal reflection, occurs. This angle defines a primary fiber specification,
the numerical aperture.
Critical angle
Figure 2-9
Total internal reflection in an optical fiber.
FIBER APPLICATIONS
Each type of fiber has its specific application. Step-index multimode fiber
is used where large core size and efficient coupling of source power are
more important than low loss and high bandwidth. It is commonly used
in short, low-speed datalinks. It may also be used in applications where
24
CHAPTER 2 — BASICS OF FIBER OPTICS
radiation is a concern, since it can be made with a pure silica core that is not readily affected by radiation.
Graded-index multimode fiber is used for data communications systems
where the transmitter sources are LEDs. While four graded-index multimode
fibers have been used over the history of fiber optic communications, one fiber
now is by far the most widely used by virtually all multimode datacom
networks—62.5/125 µm.
The telephone companies use singlemode fiber for its better performance at
higher bit rates and its lower loss, allowing faster and longer unrepeated links for
long-distance telecommunications. It is also used in CATV, since today’s analog
CATV networks use laser sources designed for singlemode fiber and future
CATV networks will use compressed digital video signals. Almost all other highspeed networks are using singlemode fiber, either to support gigabit data rates or
long-distance links.
FIBER PERFORMANCE
Purity of the medium is very important for best transmission of an optical signal
inside the fiber. Perfect vacuum is the purest medium we can have in which to
transmit light. Since all optical fibers are made of solid, not hollow, cores, we
have to settle for second best in terms of purity. Technology makes it possible for
us to make glass very pure, however.
Impurities are the unwanted things that can get into the fiber and become a
part of its structure. Dirt and impurities are two different things. Dirt comes to
the fiber from dirty hands and a dirty work environment. This can be cleaned off
with alcohol wipes. Impurities, on the other hand, are built into the fiber at the
time of manufacture; they cannot be cleaned off. These impurities will cause parts
of optical signal to be lost due to scattering or absorption causing attenuation of
the signal. If we have too many impurities in the fiber, too much of the optical
signal will be lost and what is left over at the output of the fiber will not be
enough for reliable communications.
Much of the early research and development of optical fiber centered on
methods to make the fiber purity higher to reduce optical losses. Today’s fibers
are so pure that as a point of comparison, if water in the ocean was as pure, we
would be able to see the bottom on a sunny day.
Optical glass fiber has another layer (or two) that surrounds the cladding,
known as the buffer. The buffer is a plastic coating(s) that provides scratch protection for the glass below. It also adds to the mechanical strength of the fiber
and protects it from moisture damage. On straight pulling (tension), glass optical
fiber is five times stronger than some steel. But when it comes to twisting and
bending, glass must not be stressed beyond its limits or it will fracture.
CHAPTER 2 — BASICS OF FIBER OPTICS
25
Fiber Attenuation
The attenuation of the optical fiber is a result of two factors—absorption and
scattering (Figure 2-10). Absorption is caused by the absorption of the light and
conversion to heat by molecules in the glass. Primary absorbers are residual OH+
and dopants used to modify the refractive index of the glass. This absorption
occurs at discrete wavelengths, determined by the elements absorbing the light.
The OH+ absorption is predominant, and occurs most strongly around 1000 nm,
1400 nm, and above 1600 nm.
The largest cause of attenuation is scattering. Scattering occurs when light
collides with individual atoms in the glass and is anisotrophic. Light that is scattered at angles outside the critical angle of the fiber will be absorbed into the
cladding or scattered in all directions, even transmitted back toward the source.
Scattering is also a function of wavelength, proportional to the inverse fourth
power of the wavelength of the light. Thus, if you double the wavelength of the
light, you reduce the scattering losses by 24 or 16 times. Therefore, for longdistance transmission, it is advantageous to use the longest practical wavelength
for minimal attenuation and maximum distance between repeaters. Together,
absorption and scattering produce the attenuation curve for a typical glass optical fiber shown in Figure 2-10.
Fiber optic systems transmit in the windows created between the absorption
bands at 850 nm, 1300 nm, and 1550 nm, where physics also allows one to fabricate lasers and detectors easily. Plastic fiber has a more limited wavelength band
that limits practical use to 660-nm LED sources.
Scattering
Attenuation
Absorption
850
1300
1550
Wavelength (nm)
Figure 2-10
Fiber loss mechanisms.
26
CHAPTER 2 — BASICS OF FIBER OPTICS
Fiber Bandwidth
Fiber’s information transmission capacity is limited by two separate components of
dispersion: modal (Figure 2-11) and chromatic (Figure 2-12). Modal dispersion
occurs in step-index multimode fiber where the paths of different modes are of
varying lengths. Modal dispersion also comes from the fact that the index profile of
graded-index multimode fiber is not perfect. The graded-index profile was chosen
to theoretically allow all modes to have the same group velocity or transit speed
along the length of the fiber. By making the outer parts of the core a lower index of
refraction than the inner parts of the core, the higher order modes speed up as they
go away from the center of the core, compensating for their longer path lengths.
Multimode Step Index
Cladding
Core
Multimode Graded Index
Cladding
Core
Figure 2-11 Modal dispersion, caused by different path lengths in the fiber, is
corrected in graded-index fiber.
Longer wavelength goes faster
Figure 2-12 Chromatic dispersion occurs because light of different colors
(wavelengths) travels at different speeds in the core of the fiber.
CHAPTER 2 — BASICS OF FIBER OPTICS
27
In an idealized graded-index fiber, all modes have the same group velocity
and no modal dispersion occurs. But in real fibers, the index profile is a piecewise
approximation and all modes are not perfectly transmitted, allowing some modal
dispersion. Since the higher-order modes have greater deviations, the modal dispersion of a fiber (and therefore its laser bandwidth) tends to be very sensitive to
modal conditions in the fiber. Thus the bandwidth of longer fibers degrades nonlinearly as the higher-order modes are attenuated more strongly.
The second factor in fiber bandwidth is chromatic dispersion. Remember, a
prism spreads out the spectrum of incident light since the light travels at different
speeds according to its color and is therefore refracted at different angles. The
usual way of stating this is the index of refraction of the glass is wavelength
dependent. Thus, a carefully manufactured graded-index multimode fiber can
only be optimized for a single wavelength, usually near 1300 nm, and light of
other colors will suffer from chromatic dispersion. Even light in the same mode
will be dispersed if it is of different wavelengths.
Chromatic dispersion is a bigger problem with LEDs, which have broad spectral outputs, unlike lasers that concentrate most of their light in a narrow spectral
range. Chromatic dispersion occurs with LEDs because much of the power is
away from the zero dispersion wavelength of the fiber. High-speed systems such
as Fiber Distributed Data Interface (FDDI), based on broad output surface emitter LEDs, suffer such intense chromatic dispersion that transmission over only 2
kilometer of 62.5/125 fiber can be risky.
Modal Effects on Attenuation and Bandwidth
The way light travels in modes in multimode fiber can affect attenuation and
bandwidth of the fiber. In order to model a network or test multimode fiber optic
cables accurately and reproducibly, it is necessary to understand modal distribution, mode control, and attenuation correction factors. Modal distribution in
multimode fiber is important to measurement reproducibility and accuracy.
ADVANCED STUDY
What Is Modal Distribution?
In multimode fibers, some light rays travel straight down the axis of the
fiber while all the others wiggle or bounce back and forth inside the core.
In step-index fiber, the off-axis rays, called “higher-order modes,” bounce
28
CHAPTER 2 — BASICS OF FIBER OPTICS
back and forth from core/cladding boundaries as they are transmitted
down the fiber. Since these higher-order modes travel a longer distance
than the axial ray, they are responsible for the dispersion that limits the
fiber’s bandwidth.
In graded-index fiber, the reduction of the index of refraction of the
core as one approaches the cladding causes the higher-order modes to follow a curved path that is longer than the axial ray (the “zero-order mode”).
However, by virtue of the lower index of refraction away from the axis, light
speeds up as it approaches the cladding, thus taking approximately the
same time to travel through the fiber. Therefore the “dispersion,” or variations in transit time for various modes, is minimized and bandwidth of the
fiber is maximized.
However, the fact that the higher-order modes travel farther in the
glass core means that they have a greater likelihood of being scattered or
absorbed, the two primary causes of attenuation in optical fibers. Therefore, the higher-order modes will have greater attenuation than lower-order
modes, and a long length of fiber that was fully filled (all modes had the
same power level launched into them) will have a lower amount of power in
the higher-order modes than will a short length of the same fiber.
This change in modal distribution between long and short fibers can
be described as a “transient loss,” and can make big differences in the
measurements one makes with the fiber. It not only changes the modal
distribution, it also changes the effective core diameter and apparent
numerical aperture.
The term “equilibrium modal distribution” (EMD) is used to describe
the modal distribution in a long fiber that has lost the higher-order modes.
A “long” fiber is one in EMD, while a “short” fiber has all its initially
launched higher-order modes.
In the laboratory, a critical optical system is used to fully fill the fiber
modes and a “mode filter,” usually a mandrel wrap that stresses the fiber
and increases loss for the higher-order modes, is used to simulate EMD
conditions. A “mode scrambler,” made by fusion splicing a step-index fiber
into the graded-index fiber near the source, can also be used to fill all
modes equally.
When testing the network cable plant, using an LED or laser source
similar to the one used in the system and short launch cables may provide
as accurate a measurement as is possible under more controlled circumstances, since the LED approximates the system source. Alternately, one
may use a mode conditioner (described below) to establish consistent
modal distribution for testing cables.
CHAPTER 2 — BASICS OF FIBER OPTICS
29
Mode Conditioners
There are three basic “gadgets” used to condition the modal distribution in multimode fibers: mode strippers that remove unwanted cladding mode light, mode
scramblers that mix modes to equalize power in all the modes, and mode filters
that remove the higher-order modes to simulate EMD or steady-state conditions.
These are discussed in Chapter 17.
REVIEW QUESTIONS
1. The main advantage(s) of optical is (are) its ability to ________________
than other communications media.
a. transport more information
b. transport information faster
c. transport information farther
d. all of the above
2. A fiber optic data link is made up of three elements:
1. ________________
2. ________________
3. ________________
3. Plastic optical fibers are ________________ fibers.
a. singlemode
b. large core step-index
c. large core graded-index
d. either a or b
4. Optical fiber is comprised of three layers:
1. ________________
2. ________________
3. ________________
5. What does 62.5 refer to when written 62.5/125?
a. diameter of the core
b. diameter of the cladding
c. numerical aperture
d. index profile
6. In graded-index optical fiber, the index profile approximates a parabola.
The benefit of this is ________________
a. reduced bandwidth.
b. reduced cross-talk.
c. increased modal dispersion.
d. reduced modal dispersion.
30
CHAPTER 2 — BASICS OF FIBER OPTICS
7. Three methods used to fabricate optical fiber:
1. ________________
2. ________________
3. ________________
8. Match the following fibers to the application they are best suited for:
______ Graded-index multimode a. long-distance telecommunications
______ Step-index multimode
b. data communications
______ Singlemode
c. efficient source power coupling
9. The largest cause of attenuation is ________________
a. dopants.
b. absorption.
c. moisture.
d. scattering.
10. Optical fiber’s bandwidth, or information transmission capacity, is
limited by two factors:
1. ________________
2. ________________
C H A P T E R
3
FIBER OPTIC
NETWORKS
J I M H AY E S A N D P H I L S H E C K L E R
One often sees articles written about fiber optic communications networks that
imply that fiber optics is “new.” That is hardly the case. The first fiber optic telephone network was installed in Chicago in 1976, and by 1979, commercial fiber
optic computer datalinks were available. Since then, fiber has become commonplace in the communications infrastructure.
If you make a long-distance call today, your voice is undoubtedly being
transmitted on fiber optic cable, since it has replaced over 90 percent of all voice
circuits for long-distance communications. Transoceanic links are being converted to fiber optics at a very high rate, since all new undersea cables are fiber
optics. Phone company offices are being interconnected with fiber, and most
large office buildings have fiber optic telephone connections into the buildings
themselves. Only the last links to the home, office, and phone are not fiber.
CATV also uses fiber optics via a unique analog transmission scheme, but
they are already planning on fiber moving to compressed digital video. Most large
city CATV systems are being converted to fiber optics for reliability and in order
to offer new services such as Internet connections and phone service. Only fiber
offers the bandwidth necessary for carrying voice, data, and video simultaneously.
The LAN backbone also has become predominately fiber-based. The backend of mainframe computers is also primarily fiber. The desktop is the only holdout, currently a battlefield between the copper and fiber contingents.
31
32
CHAPTER 3 — FIBER OPTIC NETWORKS
Security, building management, audio, process control, and almost any other
system that requires communications cabling have become available on fiber
optics. Fiber optics really is the medium of choice for all high bandwidth and/or
long-distance communications. Let us look at why it is, how to evaluate the economics of copper versus fiber, and how to design fiber networks with the best
availability of options for upgradeability in the future.
IT IS REALLY ALL A MATTER OF ECONOMICS
The use of fiber optics is entirely an issue of economics. Widespread use occurred
when the cost declined to a point that fiber optics became less expensive than
transmission over copper wires, radio, or satellite links. However, for each application, the turnover point has been reached for somewhat different reasons.
Telephony
Fiber optics has become widely used in telephone systems because of its enormous bandwidth and distance advantages over copper wires. The application for
fiber in telephony is simply connecting switches over fiber optic links (Figure 31). Commercial systems today carry more phone conversations over a single pair
of fibers than could be carried over thousands of copper pairs. Material costs,
Long Distance
Local Loop (City)
Subscriber Loop
(Fiber to the Curb—FTTC)
Fiber to the Home—FTTH
Figure 3-1
Telephone fiber optic architecture.
CHAPTER 3 — FIBER OPTIC NETWORKS
33
installation, and splicing labor and reliability are all in fiber’s favor, not to mention space considerations. In major cities today, insufficient space exists in current conduit to provide communications needs over copper wire.
While fiber carries over 90 percent of all long-distance communications and
50 percent of local communications, the penetration of fiber to the curb (FTTC)
and fiber to the home (FTTH) has been hindered by a lack of cost-effectiveness.
These two final frontiers for fiber in the phone systems hinge on fiber becoming
less expensive and customer demand for high bandwidth services that would be
impossible over current copper telephone wires. Digital subscriber loop (DSL)
technology has enhanced the capacity of the current copper wire home connections so as to postpone implementation of FTTH for perhaps another decade.
Telecommunications led the change to fiber optic technology. The initial use
of fiber optics was simply to build adapters that took input from traditional telephone equipment’s electrical signals on copper cables, multiplexed many signals
to take advantage of the higher bit-rate capability of fiber, and used high-power
laser sources to allow maximum transmission distances.
After many years of all these adapters using transmission protocols proprietary to each vendor, Bellcore (now Telcordia) began working on a standard network called SONET, for Synchronous Optical NETwork. SONET would allow
interoperability between various manufacturers’ transmission equipment.
However, the telephone companies’ (telco’s) transition to SONET was slow,
a result of reluctance to make obsolete recently installed fiber optic transmission
equipment and the slow development of the details of the standards. Progress has
been somewhat faster overseas, where the equivalent network standard Synchronous Digital Hierarchy (SDH) is being used for first-generation fiber optic systems. SONET is now threatened by Internet protocol (IP) networks, since data
traffic has surpassed voice traffic in volume and is growing many times faster,
mostly due to the popularity of the Internet and World Wide Web.
CATV
In CATV, fiber initially paid for itself in enhanced reliability. The enormous
bandwidth requirements of broadcast TV require frequent repeaters. The large
number of repeaters used in a broadcast cable network are a big source of failure.
And CATV systems’ tree-and-branch architecture means upstream failure causes
failure for all downstream users. Reliability is a big issue since viewers are a vocal
lot if programming is interrupted!
CATV experimented with fiber optics for years, but it was too expensive
until the development of the AM analog systems. By simply converting the signal
from electrical to optical, the advantages of fiber optics became cost-effective.
Now CATV has adopted a network architecture (Figure 3-2) that overbuilds the
normal coax network with fiber optic links.
34
CHAPTER 3 — FIBER OPTIC NETWORKS
Coax Network
Headend
Fiber Overbuild
Headend
Figure 3-2
CATV architectures before and after fiber overbuild.
Fiber is easy to install in an overbuild, either by lashing lightweight fiber
optic cable to the installed aerial coax or by pulling in underground ducts. The
technology, all singlemode with laser sources, is easily updated to future digital
systems when compressed digital video becomes available. The connection to the
user remains coaxial cable, which has as much as 1 GHz bandwidth.
The installed cable plant also offers the opportunity to install data and voice
services in areas where it is legal and economically feasible. Extra fibers can be
easily configured for a return path. The breakthrough came with the development of the cable modem, which multiplexes Ethernet onto the frequency spectrum of a CATV system. CATV systems can literally put the subscriber on a
Ethernet LAN and connect them to the Internet at much higher speeds than a
dial-up phone connection. Adding voice service is relatively easy for the CATV
operator as well.
Local Area Networks
For LANs and other datacom applications, the economics of fiber optics are less
clear today. For low bit-rate applications over short distances, copper wire is
undoubtedly more economical, but as distances go over the 100 meters called for
CHAPTER 3 — FIBER OPTIC NETWORKS
35
in industry standards and speeds get above 100 Mb/s, fiber begins to look more
attractive since copper requires more local network electronics and there are
many problems installing and testing copper wire to high speed standards. Ability to upgrade usually tilts the decision to fiber since copper must be handled very
carefully to operate at speeds where fiber is just cruising along.
Fiber penetration in LANs is very high in long-distance or high bit-rate backbones in large LANs, connecting local hubs or routers, but still very low in connections to the desktop. The rapidly declining costs of the installed fiber optic
cable plant and adapter electronics combined with needs for higher bandwidth at
the desktop are making fiber to the desk more viable, especially using centralized
fiber architectures.
There are a large number of LAN standards today. The most widely used,
called Ethernet or IEEE802.3 after its standards committee, is a 10, 100 MB/s or
1 GB/s LAN that operates with a protocol that lets any station broadcast if the
network if free. Token ring (most often referred to as IBM Token Ring after its
developer) is a 4 or 16 MB/s LAN that has a ring architecture, where each station
has a chance to transmit in turn, when a digital “token” passes to that station.
These two networks were developed originally based on copper wire standards.
Fiber optic adapters or repeaters have been developed for these networks to allow
using fiber optic cable for transmission where distance or electrical interference
justifies the extra cost of the fiber optic interfaces for the equipment.
Most LANs have been designed from the beginning to offer the option of
both copper wiring and fiber optics. Several of these networks were optimized for
fiber. All share the common specification of speed: they are high-speed networks
designed to move massive quantities of data rapidly between workstations or
mainframe computers.
Fiber Distributed Data Interface (FDDI) is a high-speed LAN standard that
was developed specifically for fiber optics by the ANSI X3T9.5 committee, and
products are readily available. FDDI has a dual counter-rotating ring topology (
Figure 3-3) with dual-attached stations on the backbone that are attached to both
rings, and single-attached stations that are attached to only one of the rings
through a concentrator. It has a token passing media access protocol and a 100Mbit/s data rate. FDDIs dual ring architecture makes it very fault tolerant, as the
loss of a cable or station will not prevent the rest of the network from operating
properly.
ESCON (Figure 3-4) is an IBM-developed network that connects peripherals
to the mainframe, replacing “bus and tag” systems. ESCON stands for Enterprise
System Connection architecture. The network is a switched star architecture,
using ESCON directors to switch various equipment to the mainframe computers. Data transfer rate started at 4.5 megabytes/second but was increased to 10
Mbytes/second. With an 8B/10B conding scheme, ESCON runs at about 200
Mbits/sec.
36
CHAPTER 3 — FIBER OPTIC NETWORKS
Counter Rotating
Primary
Node
(DAS)
Concentrator (DAC)
Secondary Nodes
(SAS or SAC)
Figure 3-3
Fiber distributed data interface (FDDI).
Mainframe
Director
Peripheral
Peripheral
Director
Director
Director
Director
Figure 3-4
Director
Enterprise system connection (ESCON) architecture.
CHAPTER 3 — FIBER OPTIC NETWORKS
37
Optically, ESCON and FDDI are similar. They use 1300-nm transmission for
the higher bandwidth necessary with high-speed data transfer rates. Both singlemode and multimode cable plants are supported and distances up to 20 kilometers between directors.
Fibre Channel and High Performance Parallel Interface (HIPPI) are both
high-speed links, not networks, that are designed to be used to interconnect highspeed data devices. The link protocol supports most fiber types and even copper
cables for some short runs.
FIBER OR COPPER? TECHNOLOGY SAYS GO FIBER, BUT . . .
Fiber’s performance advantages over copper result from the physics of transmitting with photons instead of electrons. Fiber optic transmission neither radiates
radio frequency interference (RFI) nor is susceptible to interference, unlike copper wires that radiate signals capable of interfering with other electronic equipment. Because it is unaffected by electrical fields, utility companies even run
power lines with fibers imbedded in the wires!
The bandwidth/distance issue is what usually convinces the user to switch to
fiber. For today’s applications, fiber is used at 100–200 Mb/s for datacom applications on multimode fiber, and telcos and CATV use singlemode fiber in the
gigahertz range. Multimode fiber has a larger light-carrying core that is compatible with less expensive LED sources, but the light travels in many rays, called
modes, that limit the bandwidth of the fiber. Singlemode fiber has a smaller core
that requires laser sources, but light travels in only one mode, offering almost
unlimited bandwidth.
In either fiber type, you can transmit at many different wavelengths of light
simultaneously without interference; this process is called wavelength division
multiplexing (WDM). WDM is much easier with singlemode fiber, since lasers
have much better defined spectral outputs. Telephone networks using dense wavelength division multiplexing (DWDM) have systems now operating at greater
than 80 MB/s. IBM developed a prototype system that uses this technique to provide a potential of 300 Gb/s on a LAN!
Which LANs Support Fiber?
That’s easy, all of them. Some, such as FDDI or ESCON, were designed around
fiber optics, whereas others, such as Ethernet or token ring, use fiber optic
adapters to change from copper cable to fiber optics. In the computer room, you
can get fiber optic channel extenders or ESCON equipment with fiber built in.
Where Is the Future of Fiber?
The future of fiber optics is the future of communications. What fiber optic offers
is bandwidth and the ability to upgrade. Applications such as multimedia and
38
CHAPTER 3 — FIBER OPTIC NETWORKS
video conferencing are driving networks to higher bandwidth at a furious pace.
Over wide area networks, the installed fiber optic infrastructure can be expanded
to accommodate almost unlimited traffic. Only the electronic switches need to be
upgraded to provide orders of magnitude greater capacity. CATV operators are
installing fiber as fast as possible since advanced digital TV will thrive in a fiberbased environment. Datacom applications can benefit from fiber optics also, as
graphics and multimedia require more LAN bandwidth. Even wireless communications need fiber, connecting local low-power cellular or personal communication systems (PCS) transceivers to the switching matrix.
The Copper Versus Fiber Debate
Over the past few years, the datacom arena has been the site of a fierce battle
between the fiber people and the copper people. First, almost 10 years ago, fiber
offered the only solution to high-speed or long-distance datacom backbones.
Although fiber was hard to install then and electrical/optical interfaces were
expensive, when available at all, fiber was really the only reliable solution. This
led to the development of the FDDI standard for a 100 Mb/s token ring LAN and
the IBM ESCON system to replace bus and tag cables.
By 1989, FDDI was a reality, with demonstration networks operating at conferences to show that it really worked and that various vendors’ hardware was
interoperable. In 1990, IBM introduced ESCON as part of the System 390 introduction and fiber had become an integral part of their mainframe hardware.
Everybody thought fiber had arrived.
However, at the same time, the copper wire manufacturers had developed
new design cables that had much better attenuation characteristics at high frequencies. Armed with data that their Category 5 unshielded twisted pair (UTP)
cables could transmit 100–150 Mb/s signals over 100 meters and surveys that
showed that most desktop connections are less than that distance, they made a
major frontal assault on the high-speed LAN marketplace. Simultaneously, other
high-speed LAN standards, high-speed Ethernet and asynchronous transfer mode
(ATM), which deliver FDDI speeds on copper wire, became popular. Now copper manufacturers are offering proprietary designs for copper cables that promise
250 MHz bandwidth, although the designs are years away from standardization.
Many potential users continue to postpone making the decision to go to fiber.
So How Do You Decide Between Fiber and Copper?
Some applications are really black and white. Low bit-rate LAN connections at
the desktop with little expectation of ever upgrading to higher bit rates should
use copper. Long distances, heavy traffic loads, high bit rates, or high interference
environments demand fiber. So if you have a backbone and Ethernet or token
ring on the desktop, a fiber backbone and Category 5 UTP to the desktop makes
CHAPTER 3 — FIBER OPTIC NETWORKS
39
good sense. If you already have a mainframe in the computer room and are using
channel connections, you probably will use bus and tag cables for connections.
But if you are extending those connections outside the computer room or buying
a new mainframe, you will be getting fiber optic channel extenders or ESCON.
If either media will work in your application, it really comes down to economics—which solution is more cost-effective. But cost is a combination of factors, including system architecture, material cost, installation, testing, and
“opportunity cost.”
More end users are realizing that in a proper comparison, fiber right to the
desktop can actually be significantly cheaper than a copper network. Look at the
networks (Figure 3-5), and you will see what we mean.
The Traditional UTP LAN
The UTP copper LAN has a maximum cable length of 90 meters (about 290 ft.),
so each desktop is connected by a unique UTP cable to a network hub located in
a nearby “telecom closet.” The backbone of the network can be UTP if the closets are close enough, or fiber optics if the distances are larger or the backbone
runs a higher bandwidth network than can be supported on copper. Every hub
connects to the main telecom closet with one cable per hub.
Cat 5 Copper
Fiber Optics
Horizontal
Telecom Closets
(Hub, power, UPC,
interconnections)
Fiber Patch Panels
Backbone
Main
Cross-Connect
Figure 3-5
Fiber and copper use different network architectures.
40
CHAPTER 3 — FIBER OPTIC NETWORKS
In the telecom closet, every hub requires conditioned, uninterruptable power,
since the network depends on every hub being able to survive a power outage. A
data quality ground should be installed to prevent ground loops and noise problems. It will probably also have a rack to mount everything in (and the rack must
be grounded properly.) Cables will be terminated in patch panels and patch cords
will be used to connect cables to hubs.
The Fiber to the Desk LAN
Fiber optics is not limited in distance as is UTP cable. It can go as far as 2 kilometers (over 6,000 ft.), making it possible to bypass the local hubs and cable
straight to the main telecom closet. It is likely there will be a small patch panel or
wall box connecting desktop cables (probably zipcord) to a large fiber count
backbone cable. At least 72 desktops can be connected on one backbone cable,
which is hardly larger than one UTP cable.
So an “all-fiber” fiber network only has electronics in the main telecom
closet and at the desktop—nothing in between. That means we do not need power
or a UPS in the telecom closet—we do not even need a closet! Managing the network becomes much easier since all the electronics are in one location. Troubleshooting is simpler as well.
The Myth That Fiber Is More Expensive
The myth that fiber is more expensive has been copper’s best defense against fiber
optics. In a typical cost comparison, the architecture chosen is the typical copper
one, and the cost of a link from the telecom closet to the desk, including electronics, is always higher for fiber—although by less and less each year.
But that is not a fair comparison! In a real comparison, we would price the
complete networks shown in Figure 3-5. It would look more like Table 3-1.
So what happens if we total up the costs with this comparison? One estimate
on a bank with no building construction costs had fiber costing only about $9
more per desktop. Another estimate had fiber costing only two-thirds as much as
UTP. Several new construction projects claimed saving millions of dollars by
eliminating all but one telecom closet in a large campus and thereby saving large
amounts in building construction costs.
Fiber also saves money on testing. For fiber, it is a simple matter of testing
the optical loss of the installed cable plant, including all interconnections to
worldwide standards. The test equipment costs less than $1,000 and testing takes
a few minutes per fiber.
Testing Category 5 or 6 UTP requires $3,000 to $50,000 in equipment and
very careful control of testing conditions. Standards for testing are still continuously developed to keep up with new product development. If you consider the
cost of testing, copper will probably cost a lot more than fiber!
CHAPTER 3 — FIBER-OPTIC NETWORKS
Table 3-1.
41
Comparison of Fiber and Copper Networks
Desktop
Horizontal Cabling
Telecom Closet
Backbone Cabling
Main Telecom Closet
Building
(relevant for new
construction or
major renovations)
UTP Copper
Fiber
Ethernet Network Interface
Card for Cat 5
Cat 5 cable, jacks, wall box,
patch cord
Patch panel, patch cord, rack,
hub, power
connection, UPS,
data ground
One Cat 5 cable per
connection
Patch panels, patch cords,
electronics, power, UPC
Space for large bundles of
cable, large floor or wall
penetrations, big telecom
closets, separate grounding
for network equipment
Ethernet Network Interface
Card for fiber
Fiber zipcord, connectors,
wall box, patch cord
Wall mount patch panel
One multifiber cable per
consolidation point
Patch panels, patch cords,
electronics, power, UPC
Not needed
FUTURE-PROOFING THE INSTALLATION
As fast as networks are changing, always to higher speeds, future-proofing is a difficult proposition. When the decision to install fiber is made, follow up is needed
in the planning phases to ensure that the best fiber optic network is installed.
Planning for the future is especially important. You can easily install a cable plant
for your LAN today that will fill your current needs and allow for network
expansion for a long time in the future.
Follow industry standards such as EIA/TIA 568 and install a standard star
architecture cable plant. Install lots of spare fibers since fiber optic cable is now
inexpensive, but installation labor is expensive. Those extra fibers are inexpensive to add to a cable being installed today, but installing another cable in the
future could be much more expensive.
What fibers should be installed? For multimode fibers, the most popular
fiber today is 62.5/125 micron, since every manufacturer’s products will operate
optimally on this fiber. However, most equipment is also compatible with
50/125 fiber, which has already been installed in some networks, especially military and government installations in the United States and throughout Europe.
All singlemode fiber is basically the same, so the choice is easier, although for
42
CHAPTER 3 — FIBER-OPTIC NETWORKS
most applications the specialty singlemode fibers (e.g., dispersion shifted or flattened) should be avoided.
Paying a premium for higher bandwidth or lower attenuation specifications
in multimode fibers can allow more future flexibility. Very high-speed networks
have forced fiber manufacturers to develop better fibers for gigabit networks.
Installing that fiber today may make migrating to gigabit networks easier in the
future.
How many fibers should be installed? Lots! Installation costs generally will
be larger than cable costs. To prevent big costs installing additional cables in the
future, it makes good sense to install large fiber count cables the first time; however, terminate only the fibers needed immediately, since termination is still the
highest labor cost for fiber optics.
Backbone cables should include 48 or more fibers, half multimode and half
singlemode. If you are installing fiber to the desktop, 12 fibers, again half and
half, will provide for any network architecture now plus spares and singlemode
fiber for future upgrades.
The new generation of gigabit networks may even be too fast for multimode
fiber over longer distances and they will use lasers and singlemode fiber to
achieve >1 GB/s data rates. If you want to use fiber for video or telecom, you may
need the singlemode fiber now. But you may not want to terminate the singlemode fiber until you need it, since singlemode terminations are still more expensive than multimode; however, they are getting less expensive over time.
Fiber optics has grown so fast in popularity because of the unbelievably positive feedback from users. With proper planning and preparation, a fiber optic
network can be installed that will provide the user with communication capability well into the next decade.
REVIEW QUESTIONS
1. Three areas in which fiber is used:
1. ________________
2. ________________
3. ________________
2. Match the application with the main reason fiber is the choice of transition medium.
______ LAN
a. upgradeability
______ CATV
b. reliability
______ Telecom
c. high bandwidth and distance advantages
3. FTTC stands for ________________ .
4. FTTH stands for ________________ .
CHAPTER 3 — FIBER-OPTIC NETWORKS
43
5. The development of ________________ made fiber cost-effective for
CATV applications.
a. repeaters
b. FM systems
c. AM analog systems
d. enormous bandwidth
6. Match the following LAN standards with their counterparts in the right
column.
______ Ethernet
a. dual counter-rotating ring
______ ESCON
b. most widely used LAN
______ FDDI
c. connects peripherals to a mainframe
______ Token ring
d. originally developed for copper networks
C H A P T E R
4
OPTICAL
FIBER CABLES
P AU L R O S E N B E R G
OPTICAL FIBER CABLE CONSTRUCTION
Because of the wide variety of conditions to which they are exposed, optical
fibers have to be encased in several layers of protection. The first of these layers is
a thin protective coating made of ultraviolet curable acrylate (a plastic), which is
applied to the glass fiber as it is being manufactured. This thin coating provides
moisture and mechanical protection.
The next layer of protection is a buffer that is typically extruded over this
coating to further increase the strength of the single fibers. This buffer can be
either a loose tube or a tight tube. Most data communication cables are made
using either one of these two constructions. A third type, the ribbon cable, is frequently used in telecommunications (Figure 4-1).
Loose-tube (loose-buffer) cable is used mostly for long-distance applications
and outside plant installations where low attenuation and high cable pulling
strength are required. Several fibers can be incorporated into the same tube, providing a small-size, high-fiber density construction. The cost per fiber is also
lower than for tight-buffered cables. The tubes may be filled with a gel or
wrapped in an absorbent tape, which prevents water from entering the cable and
offers additional protection to the fibers. Since these cables must be terminated
either by fusion splicing to preconnectorized pigtails or by using breakout kits,
45
46
CHAPTER 4 — OPTICAL FIBER CABLES
PVC Jacket
Kevlar (Dupont™) Strength Member
(a)
Coated Optical Fiber
Loose Tubes Containing Fibers
Inner Jacket
Outer Jacket
Central
(b) Strength Member
Inner Jacket
Outer Jacket
Region for Kevlar™ Reinforcement,
Metal Armor, etc.
Fiber Ribbons
Filler
Tube
Regions for Kevlar™
Reinforcement or
(c) Metal Armor
Figure 4-1 (a) Tight buffered fiber optic cable. (b) Loose-tube fiber optic cable.
(c) Ribbon fiber optic cable.
CHAPTER 4 — OPTICAL FIBER CABLES
47
they are more cost-effective for longer-distance applications than they are for
short-distance applications. The fibers are completely separated from the outside
environment. Therefore, the loose-tube cables can be installed with higher pulling
tensions than tight-buffered cables.
A tight-buffered cable design is better when cable flexibility and ease of
termination are a priority. Most inside cables are of the tight-buffered design
because of the relatively short distances between devices and distribution racks.
Military tactical ground support cables also use a tight-buffered design because
of the high degree of flexibility required. A tight-buffered fiber can be cabled with
other fibers, and then reinforced with Kevlar™, and jacketed to form a tightpack
(distribution) cable. Another option is to individually reinforce each fiber with
Kevlar, then jacket it. Several single fiber units can then be cabled together to
obtain a breakout-style cable where each fiber can be broken out of the bundle
and connectorized as an individual cable.
A ribbon-style cable consists of up to 12 coated fibers bonded to form a ribbon. Several ribbons can be packed into the same cable to form an ultra-highdensity, low-cost, small-size design. Over 100 fibers can be put into a 1/2-inch
square space with ribbon cables. Ribbon fibers can be either mass fusion spliced
or mass terminated into array connectors, saving up to 80 percent of the time it
takes to terminate conventional loose or tight-buffer cables.
Cable Jacketing
The materials used for the outer jacket of fiber optic cables not only affect the
mechanical and attenuation properties of the fiber, but also determine the suitability of the cable for different environments, and its compliance to various
National Electric Code (NEC) and Underwriters Laboratories (UL) requirements.
A cable that will be exposed to chemicals can utilize an inert fluorocarbon
jacket such as Kynar, PFA, Teflon FEP, Tefzel, or Halar. These materials are suitable for a very wide range of applications, although they may be too stiff for
some industrial applications.
Aerospace applications require that the cables be able to withstand a wide
temperature range and be routed through the cramped environment of an aircraft. These cables are frequently rated for continuous operation from –65°C to
+200°C, are less than 1/10 inch in size, and can sustain a bend radius of 1/2 inch.
Fire safety is a major issue. Cables used in an industrial environment, such as
a power plant, are usually placed in horizontal trays. Several cable trays may be
stacked in close proximity. In the event of a fire, both horizontal fire propagation
and the ignition of lower cable trays by the dripping of flaming outer jacket material must be prevented. An irradiated Hypalon or XLPE jacket will meet
the flame spread requirements (IEEE-383, 1974). When exposed to a flame, the
jacket material will char rather than melt and drop burning material, thus
48
CHAPTER 4 — OPTICAL FIBER CABLES
preventing the ignition of cables in lower trays. Inside premises cables have to
meet the requirements of the NEC Article 770. The outer jacket selection is essential to ensure compliance to the flame and smoke requirements.
Environmental and Mechanical Factors
Aside from buffer type, jacketing system, and flammability requirements, the
cable design also must be based on the mechanical and environmental conditions
that will be encountered throughout the system’s life span.
A cable that will be pulled through conduits, ducts, or cable trays will have to
incorporate a number of strength members and stiffening elements to add tensile
strength and to prevent sharp bends from damaging the fibers. The addition of
Kevlar increases the cable tensile strength. Kevlar can either be braided or longitudinally applied underneath the cable or fiber component jackets. The central
strength member also serves both as a filler around which the fiber components
(a)
(b)
(c)
(d)
(e)
Figure 4-2 (a) Simplex cable. (b) Zipcord cable. (c) Tightpack cable.
(d) Breakout cable. (e) Armored loose-tube cable.
CHAPTER 4 — OPTICAL FIBER CABLES
49
are cabled and as a strength member when it incorporates steel, Kevlar, or epoxy
glass rods. Another function of the epoxy glass central member is to act as an
antibuckling component, counteracting the shrinkage of the jacketing elements at
low temperatures and preventing microbends in the fibers. An epoxy glass rod
central member should always be used in cables that may be exposed to temperatures below 0°C.
Industry Standards
Physical construction of optical cables is not governed by any agency. It is up to
the designer of the system to make sure that the cable selected will meet the application requirements. However, five basic cable types (Figure 4-2) have emerged
as de facto standards for a variety of applications.
1. Simplex and zipcord: One or two fibers, tight-buffered, Kevlar-reinforced and jacketed. Used mostly for patch cord and backplane applications (Figures 4-3 and 4-4).
Coated Optical Fiber
900 uM Tight Buffer
Aramid Yarn Strength Member
PVC Jacket 3.00 MM OD
Figure 4-3
Simplex cable shown in cross-section.
Web—Thickness Approximately .015"
PVC Outer Jacket
3.00 MM Nominal Diameter
Aramid Yarn Strength Member
900 uM PVC Tight Buffer
Figure 4-4
Zipcord cable shown in cross-section.
50
CHAPTER 4 — OPTICAL FIBER CABLES
2. Tightpack cables: Also known as distribution style cables, consist of several tight-buffered fibers bundled under the same jacket with Kevlar reinforcement. Used for short, dry conduit runs and riser and plenum
applications. These cables are small in size, but because their fibers are
not individually reinforced, they need to be terminated inside a patch
panel or junction box (Figure 4-5).
3. Breakout cables: Made of several simplex units cabled together. This is a
strong, rugged design, and is larger and more expensive than the tightpack cables. Breakout cables are suitable for conduit runs and riser and
plenum applications. Because each fiber is individually reinforced, this
design allows for a strong termination to connectors and can be brought
directly to a computer backplane (Figure 4-6).
Polypropolene Binder
E-Glass Reinforced
Epoxy Rod
Nomex Core Wrap
Central Member
UP-Jacket
Optical Fiber Tight Buffer
to 900 uM
Aramid Yarn, Dupont
Kevlar™
PVC Jacketed Subgroup
Ripcord
Figure 4-5
Tightpack cable shown in cross-section.
Outer Jacket
Kevlar™ Strength Member
6 Fiber Subgroup
Central Member UP-Jacket
Central Strength Member
Figure 4-6
Breakout cable shown in cross-section.
CHAPTER 4 — OPTICAL FIBER CABLES
51
4. Loose-tube cables: Composed of several fibers cabled together, providing a small, high-fiber count cable. This type of cable is ideal for outside
plant trunking applications. Depending on the actual construction,
loose-tube cables can be used in conduits, strung overhead, or buried
directly in the ground (Figure 4-7).
5. Hybrid or composite cables: A lot of confusion exists over these terms,
especially since the 1993 NEC switched its terminology from “hybrid”
to “composite.” Under the new terminology, a composite cable is one
that contains a number of copper conductors properly jacketed and
sheathed depending on the application, in the same cable assembly as the
optical fibers. In issues of the code previous to 1993, this was called
hybrid cable.
This situation is made all the more confusing because another type
of cable is also called composite or hybrid. This type of cable contains
only optical fibers but of two different types: multimode and single
mode.
Remember that there is a great deal of confusion over these terms,
with many people using them interchangeably. It is my contention that
you should now use the term composite for fiber/copper cables, since
that is how they are identified in the NEC. And, you should probably use
hybrid for fiber/fiber cables, since the code does not give us much choice.
Central Strength Member
Outer Jacket
Inner Jacket
Kevlar™ Reinforcement
Mylar Wrap
Loose tube
Figure 4-7
Loose-tube cable shown in cross-section.
52
CHAPTER 4 — OPTICAL FIBER CABLES
CHOICE OF CABLES
The factors to be considered when choosing a fiber optic cable are:
1.
2.
3.
4.
5.
6.
7.
8.
9.
10.
11.
Current and future bandwidth requirements
Acceptable attenuation rate
Length of cable
Cost of installation
Mechanical requirements (ruggedness, flexibility, flame retardance, low
smoke, cut-through resistance)
UL/NEC requirements
Signal source (coupling efficiency, power output, receiver sensitivity)
Connectors and terminations
Cable dimension requirements
Physical environment (temperature, moisture, location)
Compatibility with existing systems
Composite Cables
If a system design calls for copper and fiber lying next to each other or in the
same conduit, the designer should consider a composite cable. This would carry a
number of copper conductors, properly jacketed and sheathed depending on the
application, in the same cable assembly as the fiber optic cable.
Installation
Although the installation methods for both electronic wire cables and optical
fiber cables are similar, there are two very important additional considerations
that must be applied to optical fiber cables:
1. Never pull the fiber itself.
2. Never allow bends, kinks, or tight loops.
In order to keep these two rules, you must identify the strength member and
fiber locations within the cables, then use the method of attachment that pulls
most directly on the strength member. By paying careful attention to the strength
limits and minimum bending radius limits and by avoiding scraping at sharp
edges, damage can be avoided.
One guideline is that the pulling tension on indoor cables should never exceed
300 pounds. Another is that the minimum bending radius of an optical fiber
cable should be no less than 10 times the cable diameter when not under tension,
and 20 times cable diameter when being pulled into place (that is, 20 times cable
diameter when under tension).
CHAPTER 4 — OPTICAL FIBER CABLES
53
Cables in Trays
Optical fiber cables in trays should be carefully placed without tugging on the
outer jacket of the cable. Care must be taken so that the cables are placed where
they cannot be crushed. Flame retardant cables are recommended for interior
installations.
Vertical Installations
Optical fibers in any type of vertical tray, raceway, or shaft should be clamped at
frequent intervals, so that the entire weight of the cable is not supported at the
top. The weight of the cable should be evenly supported over its entire length.
Clamping intervals may vary from between 3 feet for outdoor installations with
wind stress problems to 50 feet for indoor installations.
In such instances, the fibers sometimes have a tendency to migrate downward, especially in cold weather, which causes a signal loss (attenuation). This
can be prevented by placing several loops about 1 foot in diameter at the top of
the run, at the bottom of the run, and at least once every 500 feet in between.
Cables in Conduit
For all but the shortest pulls, loose-buffer cables are preferred, since they are
stiffer and their jackets generally cause less friction than tight-buffered cables.
Long pulls should be done with a mechanical puller that carefully controls pulling
tension (Figure 4-8).
The cable lubricant must be matched to the jacket material of the cable. Most
commercial lubricants will be compatible with popular types of cable jackets, but
not in every case. Lubrication is considerably more important for optical fiber
cables than for copper cables, since the fibers can be easily damaged.
Installation
In difficult installations, the cable-pulling force should be monitored with a tension meter. In these cases, the conduit should be prelubricated, and the cable
lubricated also, as it is installed. Special lubricant spreaders and applicators are
often used as well (Figure 4-9).
Except when tension meters are used, cable pulling should be done by hand,
in continuous pulls as much as possible. Often this means pulling from a central
manhole or pull box. During the pulling process, all tight bends, kinks, and twists
must be carefully avoided. If they are not, the damaged cable may need to be
removed and replaced with undamaged cable.
Two important devices to use when pulling optical fiber cables are swivel
pulling eyes and breakaway swivels. The swivel pulling eyes allow the cable to
turn independently of the pulling line or fish tape as it travels through the
54
CHAPTER 4 — OPTICAL FIBER CABLES
Figure 4-8 For long pulls, the mechanical puller applies consistent
tension and monitors it to prevent overstressing the fiber.
CHAPTER 4 — OPTICAL FIBER CABLES
(a)
55
(b)
Figure 4-9 (a) Cable lubricant can be poured directly into the conduit before
pulling. (b) For larger conduit, lubricant can be spread by pulling prepackaged
bags through the conduit. Courtesy American Polywater Corporation
conduit. Since these cables are relatively fragile, the excessive twisting that could
develop without the swivels should be carefully avoided. The breakaway swivel
works in the same way as the swivel pulling eye, except that it will pull apart
(thus stopping the pull) when the tension rises beyond a safe limit. In such a case,
the cable must be pulled back out and reinstalled with more lubricant.
Attachment
The proper method of pulling optical fiber cables is to attach the pull wire or tape
to the cable’s strength member with the correct type of pulling eye (Figure 4-10).
56
CHAPTER 4 — OPTICAL FIBER CABLES
Figure 4-10
Numerous pulling eyes are available for various types of cable.
This avoids any tension on the fibers themselves. Unfortunately, it is not always
easy to do.
When attaching to the strength members, the outer coverings are stripped
back. Care must be taken not to damage the strength members, but stripping can
normally be done with common tools. Kevlar or steel strength members can be
tied directly to the pulling eye. Other more rigid types of strength members (such
as fiberglass-epoxy) must be connected to a special set-screw device.
Indirect attachment can usually be well done with Kellems grips that firmly
grip the cable jacket. For some larger cables, this type of attachment may actually
be preferred. If you prestretch the Kellems grip and tape it firmly to the cable,
much of the cable strain will be avoided.
Indirect attachment is not desirable when the fibers will be in the path of the
forces between the pulling grip and the strength members. This is the case when
the strength member is in the center of the cable, surrounded by the fibers. In
such cases, only a small pulling force can be used.
CHAPTER 4 — OPTICAL FIBER CABLES
57
Direct Burial
Generally, only heavy-duty cables can be directly buried. Numerous hazards
affect directly buried optical fiber cables, such as freezing water, rocky soils, construction activities, and rodents (usually gophers). Burying the cables at least 3 or
4 feet deep avoids most of these hazards, but only strong metal braids or cables
too large to bite will deter the gophers.
When plowing is used as an installation means, only loose-buffered cables
are used, since they can withstand uneven pulling pressures better than tightbuffered cables. Where freezing water presents a problem, metal sheaths, double
jackets, and gel fillings can be used as water barriers.
Installation
Rather than using expensive, heavy-duty cables, 1-inch polyethylene gas pipe is
sometimes used to form a simple conduit. These tubes are also used as inner
ducts, placed inside of larger (usually 4 inch) conduits. The plastic pipes provide
a smooth passageway; by using several units inside of the larger conduit (with
spacers holding them in place), the cables stay well organized. The plastic pipe
can be smoothly bent, providing for very convenient installations and can reduce
friction for easier and longer cable pulls.
Aerial Installations
When optical fibers are to be installed aerially, they must be self-supporting or
supported by a messenger wire (See Article 321 of the NEC). Round, loose-buffer
cables are preferred and should be firmly and frequently clamped or lashed to the
messenger wire.
Cables for long outdoor runs are usually temperature stabilized. For the stabilization, steel is used if there are no lightning or electrical hazards. In other
cases, fiberglass-epoxy is used. This type of dielectric cable is preferred for high
vertical installations such as TV or radio towers.
Utilities use a special type of aerial cable called optical ground wire (OGW),
which is a power cable capable of conducting high voltages with several fibers in
the center. This type of power cable has gained acceptance with many power utilities that want communications fibers and prefer to install the OGW to get fiber
capacity almost free.
Blown-in Fiber
Another method of installing fiber is to install special plastic tubes and blow the
fibers in through the tubes using air pressure. This method does not use cable at
all, merely buffered fibers. This method is not widely used and few installations
of this type currently exist. However, it is becoming more popular since fibers can
be easily removed and replaced for upgrades.
58
CHAPTER 4 — OPTICAL FIBER CABLES
THE NATIONAL ELECTRICAL CODE
The requirements for optical fiber cable installation are detailed in Article 770 of
the NEC. There are also alternate and/or supplementary requirements in the Life
Safety Code.
Cable Designations
Remember that the NEC designates cable types differently than the rest of the
trade. The code specifies horizontal cables, riser-rated cables, and plenum-rated
cables. It also specifies cables as conductive or nonconductive. Note that a
conductive cable is a cable that has any metal in it at all. The metal in a conductive cable does not have to be used to carry current; it may simply be a strength
member.
All cables used indoors must carry identification and ratings per the NEC.
Cables without markings should never be installed as they will not pass code!
NEC ratings are:
(OFN) Optical fiber nonconductive
(OFC) Optical fiber conductive
(OFNR) or (OFCR) Riser-rated cable for vertical runs
(OFNP) or (OFCP) Plenum-rated cables for installation in air-handling
plenums
A legitimate question is whether an electrical inspector has any jurisdiction
over installations that do not use conductive cables, the fact being that such
cables do not carry any electricity. Nevertheless, such cables are dependent upon
electronic devices to send and receive their signals. In addition, the NEC does
address itself to all optical fiber cables.
Requirements
The main requirements of Article 770 are:
When optical cables that have noncurrent-carrying conductive members
contact power conductors, the conductive member must be grounded as
close as possible to the point at which the cable enters the building. If
desired, the conductive member may be broken (with an insulating joint)
near its entrance to the building instead.
Nonconductive optical cables can share the same raceway or cable tray
with other conductors operating at up to 600 volts.
Composite optical cables can share the same raceway or cable tray as
other conductors operating at up to 600 volts.
Nonconductive optical cables cannot occupy the same enclosure as power
conductors, except in the following circumstances:
CHAPTER 4 — OPTICAL FIBER CABLES
59
1. When the fibers are associated with the other conductors.
2. When the fibers are installed in a factory-assembled or field-assembled control center.
3. Nonconductive optical cables or hybrid cables can be installed with
circuits exceeding 600 volts in industrial establishments where they
will be supervised only by qualified persons.
Both conductive and nonconductive optical cables can be installed in the
same raceway, cable tray, or enclosure with any of the following:
1. Class 2 or 3 circuits.
2. Power-limited fire protective signaling circuits.
3. Communication circuits.
4. Community antenna television (CATV) circuits.
Composite cables must be used exactly as listed on their cable jackets.
All optical cables must be installed according to their listings. Refer to Section 770-53 to see the cable substitution hierarchy.
REVIEW QUESTIONS
1. Buffered fiber comes in three styles:
1. ________________
2. ________________
3. ________________
2. Loose-tube cable is used where ________________
a. ease of termination is a concern.
b. high pulling strength is required.
c. high flexibility is a concern.
d. several fibers must fit in a small space.
3. A composite cable contains ________________
a. tight-buffered cables.
b. singlemode and multimode fibers.
c. loose-tube and tight-buffered fibers.
d. copper conductors and optical fibers.
4. Match the type of cable listed with description in the right column.
______ Zipcord cable
a. contains single and multimode fibers
______ Tightpack cable
b. two fibers, tight-buffered, mostly used
______ Breakout cable
for patch cords
______ Loose-tube cable
c. contains copper conductors and optical
______ Composite cable
fiber
______ Hybrid cable
d. distribution cables
e. a small diameter, high-fiber count cable
f. several simplex units cabled together
60
CHAPTER 4 — OPTICAL FIBER CABLES
5. When pulling fiber it is best to pull on the ________________ of the
cable.
a. fiber
b. buffer tubes
c. jacket
d. strength member
6. The minimum bending radius of an optical fiber cable should be no less
than ________________ times the cable diameter when being pulled into
place.
a. 10
b. 15
c. 20
d. 25
C H A P T E R
5
SPECIFYING
FIBER OPTIC CABLE
E R I C P E AR S O N
CABLE PARAMETERS AND TYPICAL VALUES
In order to completely specify a fiber optic cable, you need to define at least 38
specifications. We divide these cable specifications into two subgroups, installation specifications and environmental, or long-term, specifications. Most of these
specifications have a standard test technique by which the parameter is tested.
Note that not all specifications apply to all situations. You will need to
review your application to determine which of the specifications in this section
are needed. For example, cable installed in conduit or in protected locations will
not need to meet crush load specifications.
INSTALLATION SPECIFICATIONS
The installation specifications are those that must be met in order to ensure successful installation of the cable. There are six such specifications:
1. Maximum recommended installation load, installation load, or installation force (in kg-force or pounds-force, or N)
2. Minimum recommended installation bend radius, installation bend radius,
short-term bend radius, or loaded bend radius (in in. or mm)
3. Diameter of the cable
61
62
CHAPTER 5 — SPECIFYING FIBER OPTIC CABLE
4. Diameter of subcable and buffer tubes
5. Recommended temperature range for installation (in degrees centigrade)
6. Recommended temperature range for storage (in degrees centigrade)
Maximum Recommended Installation Load
The maximum recommended installation load is the maximum tensile load that
can be applied to a cable without causing a permanent change in attenuation or
breakage of fibers. This characteristic must always be specified. It is particularly
important in installations that are long, outdoors, or in conduits; it is of lesser
importance when cables are laid in cable trays or installed above suspended ceilings. We present typical and generally accepted values of installation loads in
Table 5-1. Choose the value that best fits your application.
If you believe that your application will require a strength higher than those
typically specified, then you will want to specify a strength higher than those in
Table 5-1. The cost increase of specifying such a higher strength is a small percentage, typically 5 to 10 percent, of the cost of the cable.
Minimum Recommended Installation Bend Radius
The minimum recommended installation bend radius is the minimum radius to
which cable can be bent while loaded to the maximum recommended installation
load. This radius is limited more by the cabling materials than by the bend radius
of the fiber. This bending can be done without causing a permanent change in
attenuation, breakage of fibers, or breakage of any portion of the cable structure.
This bend radius is usually, but not always, specified as being no less than 20
times the diameter of the cable being bent. Specifying the bend radius is important when pulling by machine or hand through conduit, or in any long pulls.
Table 5-1 Typical Maximum Recommended Installation Loads
Application
Pounds Force
1 fiber in raceway or tray
1 fiber in duct or conduit
2 fiber in duct or conduit
Multifiber (6–12) cables
Direct burial cables
Lashed aerial cables
Self-support aerial cables
67
125
250–500
600–800
>300
>600
CHAPTER 5 — SPECIFYING FIBER OPTIC CABLE
63
In order to determine this value, you need to examine the locations in which
you are to install your cable in order to determine the bend radius to which you
will bend the cable during installation. Conversely, you can choose the cable and
specify the conduits or ducts in which you are to install the cable so that you do
not violate this radius.
Diameter of the Cable, Subcable, and Buffer Tubes
The cable must fit in the location in which it is to be installed. This is especially
true if the cable is to be installed in a partially filled conduit. It will not be important if the cable is directly buried, installed above suspended ceilings, or in cable
trays. If the diameter is limited by the space available, the diameter limits may be
the only factor that determines which of the five designs of the cable you must
choose. If cable diameter must be limited, the ribbon designs will be the smallest.
The diameter of the subcable and the buffer tube of the cable can also
become a limiting factor. In the case of a “breakout” style cable, the diameter of
the subcable must be smaller than the maximum diameter of the connector boot
so that the boot will fit on the subcable. In addition, the diameter of the element
must be less than the maximum diameter that the back shell of the connector will
accept.
Recommended Temperature Ranges for Installation and Storage
All cables have a temperature range within which they can be installed without
damage to either the cable materials or the fibers. It is more important for outdoor installations or in extreme (arctic or desert) environments and not important for indoor installations. In general, the materials of the cable restrict the
temperature range of installation more than do the fibers. Note that not all cable
manufacturers include the temperature range of installation in their data sheets.
In this case, the more conservative temperature range of operation can be used.
In severe climates, such as those in deserts and the arctic, you will need to
specify a recommended temperature range for storage (in degrees Centigrade).
This range will strongly influence the materials used in the cable.
ENVIRONMENTAL SPECIFICATIONS
The environmental specifications are those that must be met in order to ensure successful operation of the cable in its environment. There are 21 such specifications.
1. Temperature range of operation
2. Minimum recommended long-term bend radius
3. Compliance with the NEC or local electrical codes
64
CHAPTER 5 — SPECIFYING FIBER OPTIC CABLE
4.
5.
6.
7.
8.
9.
10.
11.
12.
13.
14.
15.
16.
17.
18.
19.
20.
21.
Long-term use load
Vertical rise distance
Flame resistance
UV stability or UV resistance
Resistance to damage from rodents
Resistance to damage from water
Crush loads
Resistance to conduction under high voltage fields
Toxicity
High flexibility/static versus dynamic applications
Abrasion resistance
Resistance to solvents, petrochemicals, and other chemicals
Hermetically sealed fiber
Radiation resistance
Impact resistance
Gas permeability
Stability of filling compounds
Vibration
Temperature Range of Operation
The temperature range of operation is the temperature range within which the
attenuation remains less than the specified value. Typical ranges of operation are
given in Table 5-2 for various types of applications. In general, there are very few
applications in which fiber optic transmission cannot be used solely for reasons
of temperature range of operation. In fact, some fibers have coatings that will
survive continuous operation at 400°C. For operation at such high temperatures,
fibers are usually, but not always, incorporated into a cable structure consisting
of a metal tube. For operation at exceedingly low temperatures, cables are conTable 5-2 Typical Temperature Ranges of Operation
Application
Indoor
Outdoor
Military
Aircraft
Temperature Range
(°C)
–10 to +60, –10 to +50
–20 to +60,
–40 to +50,
–40 to +70
–55 to +85
–62 to +125
CHAPTER 5 — SPECIFYING FIBER OPTIC CABLE
65
structed of plastic materials that will retain their flexibility. For cables used at less
severe temperatures (80–200°C), fluorocarbon plastics such as Teflon, Tefzel,
Kynar, and others are used.
There are two reasons for considering the temperature range of operation:
the physical survival of the cable and the increase of attenuation of the fiber when
the cable is exposed to temperature extremes.
All cables are composed of plastic materials. These plastic materials have
temperatures above and below which they will not retain their mechanical properties. After long exposure to high temperatures, plastics deteriorate, become
soft, and, in some materials, crack. Under exposure to low temperatures, plastics
become brittle and crack when flexed or moved. Obviously, under these conditions, the cable would cease to provide protection to the fiber(s).
The second reason for considering the temperature range of operation is the
increase in attenuation that occurs when cables are exposed to extremes of temperature. Optical fibers have a sensitivity to being handled. This sensitivity is seen
when the fibers are bent. This bending, which results in an increase in attenuation, is referred to as a “microbend-induced increase in attenuation.” When a
cable is subjected to temperature extremes, the plastic materials will contract and
expand at rates much greater (100 times) than those rates of the glass fibers.
This contracting and expanding results